Water-induced formation of an alkali-ion dimer in cryptomelane nanorods

TL;DR

This study reveals how water molecules influence the stability and structure of cryptomelane nanorods by promoting the formation of alkali-ion dimers, which impacts their potential use in battery electrodes.

Contribution

It provides combined experimental and theoretical evidence of water-induced K$^+$ displacement and dimer formation in cryptomelane, a novel insight into its structural chemistry.

Findings

Water displaces K$^+$ ions at high concentrations.

Formation of K$_2^+$ dimers modifies charge states.

Water alters the energetics and structure of cryptomelane.

Abstract

Tunneled metal oxides such as $\alpha-$Mn$_8$O$_{16}$ (hollandite) have proven to be compelling candidates for charge-storage materials in high-density batteries. In particular, the tunnels can support one-dimensional chains of K$^+$ ions (which act as structure-stabilizing dopants) and H$_2$O molecules, as these chains are favored by strong H-bonds and electrostatic interactions. In this work, we examine the role of water molecules in enhancing the stability of K$^+$-doped $\alpha-$Mn$_8$O$_{16}$ (cryptomelane). The combined experimental and theoretical analyses show that for high enough concentrations of water and tunnel-ions, H$_2$O displaces K$^+$ ions from their natural binding sites. This displacement becomes energetically favorable due to the formation of K$_2^+$ dimers, thereby modifying the stoichiometric charge of the system. These findings have potentially significant technological implications for the consideration of cryptomelane as a Li$^+$/Na$^+$ battery electrode. Our work establishes the functional role of water in altering the energetics and structural properties of cryptomelane, an observation that has frequently been overlooked in previous studies.